Civil Aviation Order 108.28 Instrument 2007

The central concept is the “fixed pitch wooden propeller”, meaning a propeller whose blade angles are not adjustable in service and whose structure is made from laminated timber. The order introduces several subordinate technical concepts. “Approved timbers” are those listed in CAO 108.29, and the timber must be quarter sawn unless the Director approves otherwise. “Laminae” are individual layers of timber cut from planks with the grain direction as nearly parallel to the length as possible; the slope of grain on the edges of short laminae must not exceed 1 in 12. Laminae are selected and arranged to ensure balance in the assembled block. “Side by side jointing” is permitted under certain conditions, but only plain joints may be used, and joints in adjacent laminae must be offset by at least 20 mm at the centre. The “adhesive” must be approved by the Director; resorcinol formaldehyde and epoxy adhesives prepared for timber are acceptable, while acid catalysed phenolics and urea formaldehyde types are explicitly not acceptable. “Gluing” must be performed on a flat, rigid base, with clamping starting at the centre and clamps spaced no more than 300 mm apart. After clamping, the block must rest for at least 48 hours. “Boring and drilling” require that the bore and bolt holes be straight, circular, smooth and parallel to the axis of rotation, and bolt holes must be jig drilled. Tolerances are given for bore diameter, bolt hole diameter (2.0 to 2.5 mm over nominal bolt diameter), position of bolt holes, boss face distance, counter boring diameter, and boss diameter. “Shaping” includes a mandatory storage period of at least 10 days under controlled conditions (minimum 15°C, free from draughts) after rough shaping. “Inspection” obliges the manufacturer to check angles, chords, maximum thickness, diameter, track and plan form against specified tolerances. “Protective treatment” includes lacquering (with strict temperature and film thickness requirements) or fabric covering (with approved fabric and varnish). “Metal sheathing” must be attached using approved countersunk wood screws or tinned copper rivets; localised reduction of sheath thickness by filing is prohibited, and solder cannot be used for balance correction. “Balance” must be tested at four stages, with a maximum allowable imbalance expressed in millimetre‑grammes depending on propeller diameter. Correction methods are restricted: after the first test, timber removal within tolerances; after the third test, only by application of cellulose, lacquer or varnish. “Identification markings” must be stamped between boss and root section, including drawing number, diameter and pitch, engine type, and serial number. Taken together, these concepts create a comprehensive, prescriptive manufacturing code.

The primary persons affected are manufacturers of fixed pitch wooden propellers within Australia or who supply propellers for Australian registered aircraft. This includes specialised propeller manufacturers, aircraft maintenance organisations that produce propellers under an approved maintenance organisation (AMO) scope, and possibly individual builders of amateur‑built aircraft if they manufacture a wooden propeller for their project (though such builders may be covered by specific exemptions or experimental certificates). The Director of Aviation Safety and authorised persons (CASA inspectors) are directly affected because they may direct compliance or verify it. Suppliers of timber, adhesives, fabric and metal sheathing materials are indirectly affected because their products must meet the approvals and specifications referenced by the order. For example, the adhesive must be approved by the Director, and the fabric must comply with an approved specification. Aircraft owners and operators who fit fixed pitch wooden propellers are affected because the airworthiness of their aircraft depends on the propeller being manufactured in accordance with this order. Insurers, accident investigators and legal practitioners may also need to understand the standard when determining liability or airworthiness. The order does not affect manufacturers of metal, composite or variable pitch propellers. It also does not affect importers of propellers manufactured overseas, unless CASA directs compliance with the same standard. However, in practice, foreign manufactured propellers would need to meet an equivalent or higher standard to be accepted under the aircraft’s type certificate or a supplemental type certificate. The compliance burden falls almost entirely on manufacturers, who must invest in controlled storage and gluing facilities, approved balancing equipment, jigs for drilling, and skilled labour to meet exacting tolerances. Small manufacturers or one‑off builders face disproportionate costs because the fixed overhead of setting up a compliant production line is high relative to output. The order therefore has a market concentration effect, favouring larger, established producers. CASA itself bears the administrative cost of approving timbers, adhesives and fabric specifications, and of auditing manufacturers.

The order imposes a set of prescriptive duties on any person manufacturing a fixed pitch wooden propeller in a directed context. The manufacturer must ensure that timber is from an approved source and quarter sawn (clause 2). Laminae must be cut with grain inclination not exceeding 1 in 12 on the relevant edges (clause 3.1). Laminae must be stored in the gluing room for at least 24 hours before gluing; any warped laminae must be rejected (clause 3.2). Side jointing is allowed only with plain joints and a minimum 20 mm offset between adjacent laminae at the centre (clause 3.3). The adhesive must be approved and used strictly per manufacturer’s instructions; acid catalysed phenolics and urea formaldehyde types are banned (clause 4.1 and notes). Clamping must start at the centre, with clamps spaced no more than 300 mm apart (clause 4.3). After clamping, the block must remain undisturbed for at least 48 hours, stored horizontally on one boss face if removed from the stool (clause 4.4). Boring and drilling must produce straight, circular, smooth holes parallel to the axis; bolt holes must be jig drilled (clause 5.1). Boss faces must be flat, smooth and perpendicular to the bore axis (clause 5.2). A suitable protective covering must be applied immediately after drilling to prevent exposure of end grain (clause 5.3). Tolerances specified in clause 5.4 must be met. After rough shaping, the propeller must be stored for at least 10 days at not less than 15°C and free from draughts (clause 6.1). End grain exposed during shaping must be protected immediately (clause 6.2). The manufacturer must inspect the finished propeller for angles, chords, thickness, diameter, track and plan form, meeting the tolerances in clause 7.2. Protective treatment must follow the approved drawing, with specific requirements for lacquering or fabric covering (clause 8). Metal sheathing must be fitted per the approved drawing, with prohibitions on localised reduction by filing, use of solder for balance, and damage to blade surface (clause 9). Balance must be tested at four stages; imbalance must not exceed 1,500 mm‑gms for propellers up to 2 m diameter, increasing by 750 mm‑gms per additional metre (clause 10). Correction methods are limited (clause 10.4). Finally, each propeller must be permanently marked with the drawing number, diameter and pitch, engine type and serial number (clause 11). The manufacturer has a right to seek approval from the Director for alternative timbers, adhesives or deviations, provided the approved drawing or a specific direction permits it. There is no explicit right to appeal a direction, but general administrative law remedies would apply.

CAO 108.28 does not itself create penalties. Enforcement occurs through the Civil Aviation Regulations 1988 and the Civil Aviation Act 1988. Under CAR 1988, a person who contravenes a Civil Aviation Order may be guilty of an offence. For example, regulation 2A of CAR 1988 applies the order; a breach could attract penalties of up to 50 penalty units (currently $313 per unit as of 2024-25, so approximately $15,650) for an individual, with higher amounts for bodies corporate. Alternatively, CASA may issue an infringement notice (on‑the‑spot fine) for less serious contraventions. More serious breaches may lead to prosecution in a court, with potential fines of several hundred thousand dollars for corporations. CASA may also suspend or revoke an approval or certificate that the manufacturer holds (such as a Part 145 maintenance organisation approval) if the manufacturer systematically fails to comply. The CAR 1988 also allow CASA to issue directions requiring compliance; failure to comply with a direction is a separate offence. The order’s technical detail means that a single instance of non‑compliance (e.g., using an unapproved adhesive, failing to reject warped laminae, or drilling bolt holes without a jig) could each constitute a separate contravention. Because the standards are strict and the tolerances narrow, manufacturers face a high risk of inadvertent breach. The enforcement approach appears to be based on a presumption that any deviation from the specification renders the propeller potentially unsafe, even if the deviation is minor. Manufacturers therefore operate under a liability regime that is effectively strict; there is no requirement to prove that the defect caused harm. The order also creates a private right of action indirectly: if a propeller fails and causes injury or damage, the manufacturer may face tort claims for negligence or breach of statutory duty, and compliance with CAO 108.28 would be relevant to the standard of care. Conversely, non‑compliance could be powerful evidence of fault. The enforcement framework is thus a blend of public penalty provisions and private liability.

CAO 108.28 is made under the Civil Aviation Regulations 1988 (regulation 21A), which in turn is authorised by the Civil Aviation Act 1988. It is part of a hierarchy: the Act confers ministerial and CASA powers; the Regulations provide general airworthiness and operational rules; Civil Aviation Orders fill in detailed technical specifications. This order specifically interacts with CAO 108.29, which governs the approval of timbers for wooden propellers. The order frequently refers to an “approved drawing”, which would be part of a type certificate or supplemental type certificate issued under the airworthiness provisions of CAR 1988 (Part 4A and related regulations). The drawing itself must be approved by CASA or a delegated person, and the propeller must be manufactured exactly in accordance with that drawing. This order therefore functions as a mandatory process specification that supplements the design data. It also interacts with the airworthiness requirements for maintenance: if a propeller is repaired or overhauled, the repairer may need to comply with the same manufacturing standards for any new parts incorporated. The order does not override workplace health and safety laws (e.g., model Work Health and Safety Acts); manufacturers must still comply with duties regarding safe use of adhesives, timber dust, lacquers and metalworking. The prohibition on certain adhesives may overlap with environmental or hazardous substance regulations. In addition, the order may be relevant to the Civil Aviation Safety Regulations (CASR), particularly Part 21 (certification of products) and Part 145 (maintenance organisations). However, this order predates the CASR regime (introduced in stages from 1998), so it operates as a transitional standard. Aircraft operators who fit a propeller manufactured under this order must ensure it is maintained per the maintenance manual, which will reference the manufacturing standard. Import of propellers manufactured overseas may require CASA acceptance that the foreign standard is at least equivalent. The order does not interact with trade practices or consumer law directly, but if a propeller is defective, the Australian Consumer Law may also apply. In summary, CAO 108.28 sits within a dense web of aviation safety legislation, and compliance with it alone does not guarantee overall airworthiness; the propeller must also meet all design, installation and maintenance requirements.

CAO 108.28 was originally made earlier (exact date not given) and was repealed and substituted by this 2007 instrument. The 2007 instrument is the version provided. It commenced the day after registration on 19 December 2007. The instrument states that “Civil Aviation Order 108.28 is repealed and a new Civil Aviation Order 108.28 substituted”. That suggests that the previous version may have had different content, but the source does not detail what changed. No subsequent amendments are recorded in the text. As the source is dated 2007 and the reader is likely at the current date (2025), it is possible that further amendments have occurred since; the deep dive must note that we can only rely on the instrument as supplied. Amendment history is likely limited to the initial replacement. The order has probably been consolidated into the Federal Register of Legislation, where any later amendments (such as editorial changes or updates to referenced standards) would be visible. The absence of amendment history in the source is a caveat: practitioners should check the current version on the Federal Register of Legislation before relying on this text. The instrument itself does not contain a transitional provision, savings, or expiration clause, so it remains in force until repealed or replaced. The repeal‑and‑substitute method indicates that the earlier order was entirely rewritten, rather than being amended incrementally. This is consistent with a regulatory housekeeping exercise or a response to changes in manufacturing practice or timber availability. Without the earlier version, we cannot assess whether the 2007 order relaxed or tightened requirements.

The source text of CAO 108.28 contains no mention of any court cases, administrative appeals, or tribunal decisions. There is no litigation history recorded. This is unsurprising: the order is a highly technical specification, and disputes over manufacturing standards are more likely to be resolved through CASA’s internal review processes or through enforcement proceedings that rarely reach public judgment. If a manufacturer were prosecuted for non‑compliance, the case would be heard in a federal court or magistrates court under the Crimes Act 1914 (for penalty provisions) or the Civil Aviation legislation. Such cases would typically turn on factual questions of measurement and adherence to procedures, not on interpretation of the order itself. It is possible that administrative appeals have been lodged against CASA decisions to refuse approval of a timber or adhesive, but those would be under the Administrative Appeals Tribunal (now Administrative Review Tribunal) and would not be specific to this order. In the civil liability context, a personal injury or property damage claim involving a wooden propeller failure might produce judgments that discuss CAO 108.28 as part of the applicable standard of care. However, no such cases are widely reported in Australian aviation law databases. The absence of litigation is consistent with the order’s prescriptive nature; compliance is relatively straightforward to determine, and manufacturers who follow the text are unlikely to generate disputes. Conversely, a manufacturer facing an enforcement action might challenge the validity of the order or the Director’s power to direct compliance, but such challenges are rare. Overall, the litigation history is effectively none. Practitioners should be aware that this does not mean the order is unchallengeable; it merely means no significant public case law has developed.

Several provisions contain traps for the unwary. Clause 3.2 requires laminae to be kept in the gluing room for at least 24 hours before gluing; any warped laminae must be rejected. The rejection rule is absolute, and there is no allowance for planing or flattening. Clause 3.3 restricts side jointing to plain joints only; tongue‑and‑groove or finger joints are prohibited. The offset requirement of 20 mm at the centre is easy to miss. Clause 4.1 prohibits acid catalysed phenolics and urea formaldehyde adhesives; these are common woodworking glues, and a manufacturer might inadvertently use them. The notes state that resorcinol formaldehyde and epoxy are acceptable, but only if prepared specifically for timber. Clause 4.3 mandates that clamping must start at the centre and then alternate sides; a manufacturer who clamps from one end inadvertantly will breach the requirement. Clause 5.1 requires that bolt holes be jig drilled; hand drilling is not permitted. Clause 5.3 demands immediate application of protective covering to end grain after drilling; delay could lead to moisture ingress and disqualify the propeller. Clause 6.1 imposes a 10‑day storage period after rough shaping, at a temperature of at least 15°C and free from draughts; this may be difficult to achieve in seasonal workshops. Clause 6.2 again requires immediate protection of end grain during shaping. Clause 7.2 includes tolerances for plan form that are easy to misread; the reference to “distances between the trailing edge of all sections (except that nearest the tip) and a datum plane” is complex. Clause 8.3(3) requires the lacquer film to be between 0.10 mm and 0.15 mm thick; achieving this requires careful application and measurement. Clause 9.2 prohibits localised reduction of sheath thickness by filing; a manufacturer who tries to adjust fit by filing the sheath will be in breach. Clause 9.6 prohibits the application of solder to any part of a sheath for balance; only the screw and rivet heads may be flooded with solder (clause 9.5). This is a common mistake. Clause 10.1 specifies four balance test stages; omitting any one is a breach. Clause 10.4 limits correction methods; removal of timber is only allowed after the first test, and after the third test only cellulose, lacquer or varnish may be applied. No addition of weights or solder is permitted. Clause 11.3 requires legible stamp impressions; a faint stamp could be deemed non‑compliant. The order does not contain a de minimis or “substantial compliance” provision; any deviation, however minor, is theoretically a breach. Manufacturers should also be aware that the order applies only when directed; but in practice, any propeller intended for a certified aircraft will be directed to comply. The cumulative effect of these prescriptive rules creates a high compliance burden, and small oversights can make the entire propeller non‑airworthy.

Compliance involves a systematic, documented process that mirrors the order’s clauses. First, obtain the approved drawing for the propeller model. Source timber from a CAO 108.29 approved list, and ensure it is quarter sawn unless a specific approval for another sawing method has been granted. Cut laminae from the planks, checking grain inclination does not exceed 1 in 12 on edges of short laminae. Reject any laminae that are warped. Store laminae in the gluing room for at least 24 hours. Use only an adhesive approved by the Director (resorcinol formaldehyde or epoxy for timber are acceptable; avoid phenolics and urea formaldehyde). Follow the adhesive manufacturer’s instructions exactly. Prepare a flat, rigid gluing base that supports the entire board length. Apply adhesive to laminae, assemble the block, and start clamping at the centre, then alternate clamps left and right, spacing them no more than 300 mm apart. After clamping, leave the block undisturbed for at least 48 hours. If removed from the stool, store horizontally on one boss face. After curing, bore the centre bore and drill bolt holes using a jig; ensure holes are straight, circular, smooth and parallel to the axis. Check boss faces are flat and perpendicular. Immediately apply a protective covering to exposed end grain. Meet the tolerances in clause 5.4. Shape the propeller roughly, then store it for at least 10 days at a temperature not less than 15°C, free from draughts. Protect end grain exposed during shaping. During final shaping, inspect for faults. After shaping, carry out a detailed inspection of angles, chords, maximum thickness, diameter, track and plan form, using the tolerances in clause 7.2. Apply protective treatment per the approved drawing: if lacquering, ensure the shop temperature is 20°C to 30°C, dry and free of draughts; apply lacquer to a film thickness of 0.10 mm to 0.15 mm, free of bubbles and pin holes. If fabric covering, use approved fabric, apply in a single piece, varnish after drying, and follow the drawing. Fit metal sheathing if specified: use approved countersunk wood screws (plated) or, where blade thickness is less than 10 mm, 14 SWG tinned copper rivets. Do not reduce sheath thickness by filing. Punch or spin edges into the timber to form countersinks; avoid bulging or buckling. Clean screw slots before insertion. Flood screw and rivet heads with soft solder. Do not apply solder elsewhere for balance. Provide drainage slots in tip segments. Balance the propeller at four stages: after final shaping, after sheathing, in finished state (save for balance correction), and after any corrections. Use an approved balancing stand. Ensure imbalance does not exceed the limits in clause 10.3. Correct imbalance only by removing timber within tolerances (after first test) or by applying cellulose, lacquer or varnish (after third test). Do not use any other method. Finally, stamp the propeller between boss and root section with the drawing number, diameter and pitch (preceded by D and P), engine type, and serial number. Ensure stamps are legible and will not be obscured by the hub flange. Retain records of each compliance step, including timber certificates, adhesive batch numbers, temperature logs, dimensional inspection results, and balance test reports. Regular calibration of measuring equipment (callipers, protractors, balancing stand) is essential. A compliance checklist based on the order’s clauses can help avoid omissions. If any deviation from the order or the approved drawing is necessary, seek prior approval from the Director. Periodic internal audits and, if possible, external CASA audits will verify continued compliance.